1. Field of the Invention
The present invention relates to a vehicle control device and a vehicle control method for allowing cranking of an engine by increasing clutch torque when there is a deceleration request during free running.
2. Description of the Related Art
Technology of detecting negative pressure (vacuum) of a brake booster during idling stop and restarting an engine when the negative pressure of the brake booster is lower than a predetermined threshold is conventionally known. According to such technology, it is possible to always keep the negative pressure of the brake booster at an optimal value and inhibit a driver from feeling discomfort at the time of brake operation due to insufficient negative pressure of the brake booster.
In the above-described conventional technology, it is required to set a threshold of the negative pressure of the brake booster at which the engine is restarted to a high value to some extent in consideration of various road surfaces such as a slope with steep gradient. That is to say, the negative pressure higher than that on a flatland is required on the slope with the steep gradient, so that it is required to set the threshold to the high value for the slope with the steep gradient.
However, when the threshold is set high, the negative pressure of the brake booster is increased by unnecessary restart of the engine even on the flatland on which the vehicle may be stopped without a sense of discomfort with the small negative pressure. Therefore, frequency of the restart of the engine increases and fuel consumption decreases.
From such a background, technology of decreasing the threshold of the negative pressure of the brake booster when the vehicle stops on the flatland as compared to a case in which this stops in a place other than the flatland is suggested (refer to Japanese Laid-open Patent Publication No. 2006-83830).
Technology of restarting an engine by performing push starting to increase an engine speed by engaging a clutch when restarting operation of the engine is performed during idling stop while running (free running) is known (refer to Japanese Laid-open Patent Publication No. 2012-172578). Therefore, it is considered to secure negative pressure of a brake booster by restarting the engine by the push starting when a driver performs decelerating operation during the free running. However, required braking force is larger in general while the vehicle runs than that when the vehicle stops. Therefore, even when the engine is restarted by the push starting, the negative pressure of the brake booster might be insufficient and the driver might feel that the braking force is insufficient. That is to say, there is a room for improvement of a method of restarting the engine when the negative pressure of the brake booster is decreased during the free running.
There is a need for a vehicle control device and a vehicle control method capable of inhibiting the driver from feeling that the braking force is insufficient when the driver performs the decelerating operation during the free running.